Capacitive measuring transducer

ABSTRACT

PCT No. PCT/AT80/00027 Sec. 371 Date Mar. 27, 1981 Sec. 102(e) Date Mar. 27, 1981 PCT Filed Aug. 18, 1980 PCT Pub. No. WO81/00620 PCT Pub. Date Mar. 5, 1981.Capacitive measuring transformer, especially for small capacitance values, with two parallel bridge arms, connected with an oscillator, each of which has at least one capacitor, whereby a differential amplifier is provided which is connected on the input side with the bridge arms and the input of which is connected by a feedback circuit, containing passive elements only to one of the bridge arms. A control element (D1, D2) precedes every one of the bridge arms and a fixed potential or a potential changing inversely in relation the outlet of said differential amplifier, (OA1) is applied to the second bridge arm. As a result, one will moreover achieve a high measure of stability and linearity between capacitance and output voltage. The fixed potential is used when the feedback is to one bridge arm only. The potential changing inversely in relation to the output of the differential amplifier (OA1) is used when the feedback to both bridge arms gives the highest degree of stability and linearity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to capacitive measuring transducer apparatus, especially for small capacitive values, and more particularly to such apparatus with two bridge arms, each of which has at least one capacitor, and incorporating a differential amplifier connected on the input side with the bridge arms, each possibly having an amplifier, and the output of the differential amplifier is fed back to one of the bridge arms.

2. Prior Art

Such measuring transducer apparatus is disclosed in German OS No. 2 641 758. In case of this known measuring transducer, the two bridge arms, each of which has an amplifier, in the control circuit of which there is a capacitor or a parallel connection of capacitors and in the main circuits of which there always is a resistance combination for interconnecting terminal points of the bridge arms and an oscillator. The evaluation of capacitance changes of one or two of the capacitors disposed in different bridge arms takes place in the known case by way of the amplifiers acting as charge amplifiers, -which may be, for example, d.c. current operational amplifiers with a capacitive feedback, which may produce an output voltage, which is directly proportional to the change of the charge at the input connections. The differential amplifiers evaluate the output signals, to which a filter and a phase demodulator is series connected, the second input of which is connected with the oscillator, as well as a differential amplifier series connected to the phase demodulator, and a wave-band filter. The output voltage of the wave-band filter, at the same time, is fed back to a bridge arm via an analog multiplier, which precedes the latter.

The disadvantage of this known transducer lies in the fact that the individual parts of the circuit are very expensive and that the consequent costs are very high, to which the analog-multiplier also contributes considerably. Another disadvantage of this transducer, which operates according to the carrier-frequency method, also results from the fact that a sinusoidal generator is needed, in case somewhat higher demands are made, and this likewise results in a considerable expense.

SUMMARY OF THE INVENTION

It is the goal of the invention to propose a transducer of the initially mentioned type which is distinguished by a simple construction and nevertheless has a high degree of stability.

This will be achieved according to the invention by connecting a control element in series with each bridge arm. One of the terminals of the control elements is connected together and this common point is connected to the output of a square wave generator. The other terminals of the control elements, along with their respective bridge arms are each connected through at least one passive circuit element to a potential source. A feed-back circuit is provided, consisting of essentially passive circuit elements and which is connected to one of the capacitors in one of the bridge arms, whereas the corresponding capacitor in the other bridge arm is connected, via a passive circuit element to a fixed potential or to a potential which is determined by the differential amplifier and which varies in an inverse manner with the output of the differential amplifier.

The potential at the bridge arms is changed by way of the control elements by the pulse train delivered from the oscillator, in correspondence with the pulse train, whereby potential changes on the electrodes of the capacitors occur, and charge transferring currents develop in the bridge arms, corresponding to the capacitance values of the capacitors. Differences in the currents of the bridge arms or voltage differences at certain points of the two bridge arms are amplified by the differential amplifier and are fedback to a bridge arm via the feedback circuit to influence the extent of the potential variations on the basis of the pulse train coming from the oscillator in one bridge arm, whereas the second bridge arm is influenced during the off period of the control elements by a fixed potential or the differential output of the differential amplifier.

As a result it will be possible to make do with a very simple construction, whereby a very high stability and linearity will be achieved by a correspondingly strong feedback.

According to another characteristic of the invention provision has been made that the control elements are made up by diodes and the pulse train delivered by the oscillator fluctuates between fixed values which lie above and below the potential of the connecting points of the diodes with the bridge arms, shifted by the forward voltage of the diodes, which results in a particularly simple construction.

Basically, however, it is also possible, to provide transistor or controlled rectifiers at one of the terminal points of the bridge arms instead of the diodes and to connect them to a d.c. voltage source serving as an auxiliary voltage, whereby then these units are acted upon by an additional auxiliary voltage source, supplying a pulse train consisting of rectangular pulses. An alternating change of the charges of the capacitors of the bridge will likewise be achieved, whereby the changes in capacitance of a capacitor, as has been described already, likewise effect a change of the relationship of the currents flowing through the two bridge arms, which leads via the amplifiers and the differential amplifier to a change of its output voltage.

Since the bridge along with the amplifiers are turned on and off practically in the rhythm of the through connection of the control elements connecting the two bridge arms with the auxiliary source of voltage, high common mode variations result at the outputs of the amplifiers or at the inputs of the differential amplifier. In order to avoid these variations, provision has been made advantageously for the inputs of the differential amplifier to be always connected with a capacitor, whereby the two other electrodes of these capacitors are connected jointly to a fixed potential, as a result of which the common mode variations are dampened or smoothed very greatly and therefore one will be able to make do with a very much more simply structured differential amplifier.

A particularly extensive suppression of the common mode variations results whenever the inputs of the differential amplifier are each connected with a capacitor, whereby the two other electrodes of these capacitors are connected jointly with a potential divider, connected to a fixed potential, the central connection of which is connected via a diode with the oscillator. By this measure it will be possible to achieve a very high damping of the common mode variations even with very small capacitors.

In a preferred embodiment of the invention provision has been made that the control inputs of two amplifiers, preferably formed by transistors, are connected to the connecting points of the bridge arms, each of said bridge arms consisting preferably of two capacitors, and that the outputs of the amplifiers are connected with the differential amplifier. As a result it will be possible to use small capacitors. Thus the possibility results of using an auxiliary voltage with high frequency, supplied by the oscillator, as the result of which, capacitive changes occurring very rapdily are still determinable.

According to a further characteristic of the invention provision has been made that the connection points of the individual bridge arms connected with the control inputs of the amplifiers, preferably made up by transistors, are connected via diodes and when required, parallel connected resistors with a fixed potential. In this way one will achieve a limitation of the inverse emitter basis voltages of the transistors, whereby, as a result of a parallel connection of resistances to these limiting diodes, possible differences in their characteristics remain practically without influence on the measurement.

In order to increase the linearity between the output signal of the converter or of the differential amplifier and of the capacitance, it will be of advantage, whenever the emitters of the transistors connected with their bases with the connecting point of each of the assigned bridge arms, are each connected via one resistance with another resistance connected to a fixed potential.

In a measuring circuit, in which an amplifier is disposed in each bridge arm, and the differential amplifier is connected on the input side with the main current path of these amplifiers, provision has been made, according to another characteristic of the invention, for the emitter of the transistors provided as amplifiers to be connected with resistances, which are series connected preferably with condensers and with their connected discharge resistances and the emitters and the bases of these transistors are connected via resistances to a fixed potential, whereby the bases of the transistors are connected with these capacitors of the bridge arms, which are connected with the control elements. In case of such measuring circuits it is possible to provide very small capacitances in the base circuit of the transistors in connection with low resistance discharge resistances, so that small time constants result and a rectangular pulse sequence with high frequency may be applied to the control elements. As a result it will be possible to determine even very short duration changes in capacitance, especially whenever one or two variable capacitors lie in the main current path of the transistors.

In case of such a measuring circuit it will furthermore be of advantage, whenever the second electrodes of the capacitors lying in the main current path of the transistors and the discharge resistances are connected to a joint fixed potential, whereby the potential connected with the resistances is more negative than the inputs of the differential amplifier. By this latter measure, one will achieve that the level of the common mode voltage at the inputs of the differential amplifier will remain far enough removed from the potentials of its supply voltage. A particularly high measure of linearity between the output voltage and the capacitance to be measured results, whenever the output of the differential amplifier is fed back directly and via an inverter to the bridge arms. The same may also be achieved by feedback of a differential output of the differential amplifier to the second bridge arm via passive circuit elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, advantages and features of the invention will now be explained in more detail on the basis of the following description of preferred embodiments of the best mode of carrying out the invention in conjunction with the drawings, wherein:

FIG. 1 shows an embodiment with voltage tap on the bridge arms, and

FIG. 2 shows an embodiment with amplifiers inserted into the bridge arms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the case of the embodiment according to FIG. 1, the bridge arms are formed by capacitors C₁, C₃ and C₂, C₄ whereby one electrode of the capacitors C₃, C₄ is jointly connected to a fixed potential, for example, ground. The two other ends of the two bridge arms are connected via the resistor R₆ to a potential e₁, and via the resistor R₇ to a potential e₂, whereby the potential e₂ is derived from the output voltage of the measuring circuit for the purpose of feedback via the potentiometer P₁. Furthermore, these ends of the bridge arms are connected via the diodes D₁ and D₂ with an auxiliary source of voltage not shown, which delivers a pulse train consisting of rectangular impulses.

The connecting points of the two capacitors C₃, C₁ and C₂, C₄ constituting respective bridge arms are respectively connected with the bases of the two transistors Q₁, Q₂, combined preferably in a small integrated pair, which serve as amplifiers. Furthermore, these connecting points are applied via diodes D₃, D₄ and via resistors R₂, R₃ connected in parallel to the former, to a fixed potential V+. The resistor R₁ with which the emitters of the two transistors Q₁, Q₂ are connected via additional emitter resistors R₁ ', R₁ ", is connected also to this potential. Furthermore, the joint connecting point of two capacitors C₅, C₆, serving for the common mode variation limitation of the differential amplifier OA₁ is applied to this potential. These capacitors are connected with the collectors of the transistors Q₁, Q₂ along with the inputs of the differential amplifier OA₁. Furthermore, the collectors of the transistors Q₁, Q₂ along with inputs of the differential amplifier OA₁ are connected with resistors R₄, R₅ which are jointly connected to a fixed potential V-.

This circuit has two operational states: The first state, which in the following will always be called the basic position, occurs whenever the pulse input is high ("high" means a voltage more positive than either the potential e₁ or the potential e₂). After some time in this state practically no current flows into the capacitance bridge. The diodes D₁ and D₂ are in the blocking state. One of the electrodes of capacitor C₁ is at the potential e₁, one of the electrodes of capacitor C₂ is at the potential e₂. The potential e₂ will be determined by the output voltage and the adjustment of the potentiometer P₁. The center points of the bridge and therefore also the bases of the two transistors Q₁, Q₂ are at the potential of one of the poles V+ of a d.c. voltage source through resistors R₂ and R₃. Since the emitters of the transistors Q₁, Q₂ are also connected with the same pole V+ via the resistors R₁ ', R₁ " and R₁, the transistors Q₁, Q₂ block, and the capacitors C₅, C₆ are being charged. These capacitors C₅, C₆ are interconnected and are also connected to a voltage divider made up by the resistors R₁₀, R₁₀ ', which divider is connected to a fixed potential and the center tap of which is connected via a diode D₅ with the pulse input E or with the oscillator. The second state of the circuit which in the following will be called the active position, occurs whenever the pulse input is set on LOW. "LOW" means a voltage considerably more negative than the potential e₁, e₂. The diodes D₁ and D₃ become conductive. The one electrode from capacitors C₁ and C₂ drops down to a voltage e₃, whereby e₃ is a potential which lies by one diode forward voltage higher than the potential existing at the pulse input. Because of the steep negative edge of the input pulses a relatively great current flows for a short time through the capacitor bridge C₁, C₃, C₂, C₄ and through the bases of the two transistors Q₁, Q₂. These base currents cause reinforced collector currents which cause corresponding drops in voltage via the resistors R₄, R₅, whereby the potentials existing as a result on the collectors of the transistors Q₁, Q₂ are fed to the inputs of the differential amplifier OA₁. The common mode variation at the inputs of the differential amplifier OA₁, resulting at the moment of connecting-through, is dampened by the capacitors C₅, C₆ and by the resistors R₁₀ and R'₁₀ and the diode D₅. At the same time the positive voltage jump resulting in the moment of connecting-through of the transistors Q₁, Q₂ with interconnected electrodes of the capacitors C₅, C₆ leads to a connecting through of the diode D₅ to the pulse input E, where at this time a LOW-signal is applied. First of all it is assumed that the capacitance of capacitors C₁ C₂, C₃ and C₄ are equal and that voltage e₂ equals voltage e₁. In this case the charge transmissions on both sides of the bridge are equal, and the two transistors Q₁, Q₂ cause equally large potential changes at the inputs of the differential amplifier as soon as they have become conductive on the basis of a "LOW" signal existing at the pulse input.

Whenever the capacitance of capacitor C₃ does not equal the capacitance of capacitor C₄ and voltage e₂ equals voltage e₁, or whenever the capacitance of capacitor C₃ equals the capacitance of capacitor C₄ and voltage e₂ does not equal voltage e₁, the charge changes are unequal and the potentials taken at the collectors of the transistors Q₁, Q₂ or the resistors R₄, R₅ become unequal, as a result of which the output voltage of the amplifier OA₁ changes. The circuit remains in this active state for only a relatively short time and the positive going leading edge of the input pulse again returns the circuit into its basic state. The diodes D₃ and D₄ limit the inverse emitter base voltages and the parallel connected resistances R₂ and R₃ avoid the effect on the measurement of possible differences in the characteristic curves of the diodes D₃, D₄.

The pulse frequency will effectively be adjusted as high as possible, but with consideration to the time needed for the circuit to stabilize itself in its basic state. The resistors R₄ and R₅ are proportional such that the inputs of the amplifier OA₁ remain within its common mode voltage range, and that the collectors of the two transistors always have a negative potential vis-a-vis the emitters. The values C₅ and C₆ are so great, that the common mode variation remains small after each negative input pulse.

The drawing shows, that one side of the two capacitors C₃ and C₄ is connected to a constant potential, for example, ground, i.e., C₃ and C₄ may represent a differential capacitor. This is the case with many capacitive transducer types. Whenever C₁ equals C₂ and C₃ and C₄ form a transducer, then it will be possible to measure very small changes of capacitance, whenever the amplifier OA₁ has a high open loop amplification, whereby the circuit is kept stable by inverse feedback from the output. In this connection, the invention shows in the case of such measuring transducers for the first time the possibility for the arrangement of an inverse feedback formed merely by resistances.

The potential e₁ may be a constant potential (even zero) or it may be fed by an inverse feedback from the amplifier OA₁, whenever the latter has a differential output. Furthermore, the potential e₁, as indicated by a broken line may be fed by the tap of the potentiometer P₁ via an inverter I₁, wired with the resistances R₃₀, R₃₁. In the latter case a particularly good linearity results between the capacitance of the capacitors C₃, C₄ and the output voltage of the differential amplifier OA₁, whereby the extent of the inverse feedback is easily adjustable.

It is also possible to use transistors instead of the diodes D₁ and D₂, which are acted upon at their bases together with a pulse train, preferably consisting of rectangular impulses, in order to establish alternatingly a connection between the capacitors C₂ or C₁ and a fixed potential the height of which must naturally be different from that of the potentials e₁ and e₂.

Another change of the illustrated circuit is also possible in such a way, that a differential amplifier OA₁ with differential output is used, and the potential e₁ is tapped by the second of the differential outputs.

The following is a Table of values for certain of the components shown in FIG. 1.

    ______________________________________                                         R.sub.1 =                                                                                15 KΩ                                                                             R.sub.6 =                                                                               10 KΩ                                                                          C.sub.5 = 0.01 μF                         R.sub.1 ' =                                                                              10 KΩ                                                                             R.sub.7 =                                                                              10 KΩ                                                                           C.sub.6 = 0.01 μF                         R.sub.1 " =                                                                              10 KΩ                                                                             R.sub.30 =                                                                             22 KΩ                                         R.sub.2 =                                                                                27 KΩ                                                                             R.sub.31 =                                                                             22 KΩ                                         R.sub.3 =                                                                                27 KΩ                                                                             P.sub.1 =                                                                              10 KΩ                                         R.sub.4 =                                                                              330 KΩ                                                                              C.sub.1 =                                                                              22 pF                                               R.sub.5 =                                                                              330 KΩ                                                                              C.sub.2 =                                                                              22 pF                                               ______________________________________                                    

When the above values are used, the resistances R₁₀ and R₁₀ ' are equal to zero and the diode D₅ is eliminated.

C₃ and C₄ are each respective halves of a differential capacity transducer. In the center position each half has a capacity of approximately 10 pF.

The diodes D₁, D₂, D₃ and D₄ can be general purpose diodes such as 1N914 or, for better zero point stability, D₁ and D₂ as well as D₃ and D₄ can be diode pairs such as type BAV 70 from Motorola. The transistors Q₁ and Q₂ are preferably paired transistors on a single chip such as type 2N4937 from Motorola.

The amplifier OA₁ and the inverter I₁ are conveniently halves of a double amplifier such as type LF 353 from National Semiconductor.

V⁺ and V⁻ are ±15 volts and serve also as power supplies for the LF 353.

The pulse input at point E has the following characteristics:

The amplitude can vary from +3 V to +15 V and to -15 V. However, the conversion factor, output-volts/capacity change, is dependent on the negative voltage and it should therefore be stable.

The negative going pulse edge should have a slope of approximately 300 V per micro-second. The positive going pulse edge is not important but if its slope is greater than 20 V per micro-second, then a frequency of approximately 200 KHz can be used when the source is a square wave generator. This frequency can be increased if the dwell time at -15 V is made shorter than the positive dwell time. For instance, a dwell time of 0.5 micro-second at -15 volts and a repetition rate of 300 to 400 KHz. The dwell times should be long enough for the bridge currents to sink to practically zero. When this is the case then the conversion factor, output-volts/capacity change, is not sensitive to small changes in slope, pulse width or repetition rate, due to the stabilizing effect of the negative feed-back at points e₁ and e₂.

With P₁ so adjusted that approximately 10% of the output voltage is fed back to the points e₁ and e₂, the conversion factor is approximately 10 volts output per pico-farad of bridge unbalance.

In case of the embodiment of FIG. 2, the two bridge arms are made up by the capacitor C₃₁, the emitter resistor R₁₁ and the capacitor C₃ or the capacitor C₃₂, the emitter resistor R₁₂ and the capacitor C₄. The transistors Q₁, Q₂ at the same time, are connected to the bridge arms as amplifiers, and are connected with their collectors with the inputs of the differential amplifier OA₁. The capacitors C₃, C₄ in the main current paths of the transistors Q₁, Q₂ are connected with discharge resistors R₅₁, R₆₁ which, however, are connected to another fixed potential -V than the latter themselves, whereby the potential connected with the resistors R₅₁, R₆₁ is more negative than the potential applied at the inputs of the differential amplifier OA₁, and thus the common mode voltage at its input remains sufficiently far removed from the potentials of its supply voltage. Furthermore, the emitters and the bases are connected with the same potential V, via the resistors R₅₁, R₂₁, or R₆₁, R₂₂. At the same time a modification of the circuit would also be possible, according to which the resistors R₂₁, R₂₂ are connected with one of their terminals with the resistors R₁₁ and R₅₁ or R₁₂ and R₆₁ instead of with the potential V, and thus would be connected in parallel to the diodes D₃ or D₄, which limit the inverse-emitter-basis voltage.

The remaining part of the circuit, especially the feedback of the output of the differential amplifier OA₁ via the potentiometer P₁ and the resistor R₇ to the one bridge arm is identical to the circuit according to FIG. 1. Merely the capacitors C₅, C₆ provided at the inputs of the differential amplifier OA₁ for damping the common mode variation, are connected directly to a fixed potential instead of to the pulse input E via a diode D₅. FIG. 2, otherwise indicates the possibility of the feedback of a differential output of the differential amplifier OA₁ to the second bridge arm via the potentiometer P₂ by a broken line.

The potential of the pulse train f, arriving at the input E, changes between values, which lie above and below the potentials e₁ or e₂, at the capacitors C₃₁, C₃₂, shifted by as much as the forward voltage of the diodes D₁, D₂.

Whenever there is a negative potential at input E, then the diodes D₁ and D₂ lock and the one electrodes of the capacitors C₃₁, C₃₂ take on the potential e₁ or e₂, and then no current flows in the circuit until a positive going leading edge of the pulse train f terminates the basic state. In that case, the diodes D₁, D₂ become conductive and the potential variation is transferred via the capacitors C₃₁, C₃₂ to their electrodes connected with the bases of the transistors Q₁, Q₂ and of the resistors R₂₁ or R₂₂, so that currents flow via these resistors, of which the ones flow off via the bases of the assigned transistors Q₁, Q₂ and connect them through. Despite the more positive potential at input E, that of the electrodes of the capacitors C₃₁ or C₃₂ connected with the bases and the resistors R₂₁ or R₂₂, decreases and on the other hand the potential of the electrode of the capacitor C₃ or C₄, connected with the assigned transistor Q₁ or Q₂ via the emitter resistors R₁₁ or R₁₂ increases, as a result of which the base voltage of the transistors Q₁, Q₂ decreases and the latter become non-conductive. The emitter resistors R.sub. 11, R₁₂ moreover prevent the transistors Q₁, Q₂ from being saturated. In any case, the non-conduction of the transistors Q₁, Q₂ occurs, whenever a negative edge of the pulse train f arrives at input E, since then the potential of the one electrode of the capacitors C₃₁, C₃₂ drops to the level of the potentials e₁ or e₂ and this potential variation is transferred to the second electrodes and thus to the bases of the transistors Q₁, Q₂. The diodes D₃, D₄ in this case limit the base-emitter inverse voltages.

In case of the non-conductivity of transistors Q₁, Q₂ the capacitors C₃, C₄ are discharged via the resistors R₅₁ or R₆₁ and partly also via the diodes D₃ or D₄ and the resistors R₂₁ or R₂₂, whereby the potential of the electrodes of the capacitors C₃₁, C₃₂ connected with said resistors after their negative potential change, conditional on the negative edge of the pulse train is again raised, so that the circuit quickly passes over into its basic state, in which practically no current flows.

Differences in the currents flowing over the emitter collector sections of the transistors Q₁, Q₂ are amplified by the differential amplifier OA₁ connected with these sections. Whenever the capacitance of one of the capacitors C₃, C₄ changes, or-in case these are the part-capacitors of a differential capacitor-of both capacitors, then the consequence of this is a change of the currents, flowing via the emitter-collector sections of the transistors Q₁, Q₂, as a result of which the output signal of the differential amplifier OA₁ also changes. Thus the potential also changes, however, which during the basic state of the circuit was applied to the electrode of the capacitor C₃₂, connected with the diode D₂, since this electrode is connected with the output A, via the feedback, consisting of the resistor R₇ and the potentiometer P₁. This in turn, influences the size of the potential variation, conditional on the next positive edge of the pulse train, which in turn determines the size of the base current, flowing off into the base of the transistor Q₂, which on its part determines the size of the current, flowing via the emitter-collector section. The potential e₁ applied during the basic state of the circuit of the capacitor C₃₁, with which said capacitor is connected via the resistor R₆, on the contrary remains unchanged or is taken off by a second differential output of the differential amplifier OA₁. A further possibility consists in the fact of connecting the output of the differential amplifier OA₁ by way of feedback to the second bridge arm via an inverter, as shown in a broken line in FIG. 1.

In this way the circuit is kept in equilibrium, whereby one will get along with a very low feedback energy, since it does indeed act on the base circuit.

As a result of the two capacitors C₆, C₅ connected to a fixed potential and lying in parallel to the inputs of the differential amplifier OA₁ the common mode variation at the inputs of the differential amplifier is suppressed or decreased.

In case of the circuit of FIG. 2, various changes are conceivable. Thus, the resistors R₆ and R₇ may also be connected with the electrodes of the capacitors C₃, C₄, connected with the emitter resistors R₁₁, R₁₂ of the transistors Q₁ or Q₂, instead of with the diodes D₁ or D₂. In this case, this would result, during the basic state of the circuit, in a charge of the capacitor C₄, dependent on the output voltage of the differential amplifier OA₁, which again influences the current flowing via the transistor Q₂ during the active phase of the circuit. In case of this variation, fixed potentials would have to be applied via resistances to the capacitors C₃₁, C₃₂ or the diodes D₁, D₂.

In case of the circuit as in FIG. 2, the capacitors C₃₁, C₃₂ may have very small values, as a result of which very small time constants become possible and one will be able to operate with a pulse train delivered by an oscillator, not shown, in the MHz-area, as a result of which changes of capacitance of the measuring capacitors C₃, C₄, taking place very quickly may be determined.

A further variation of the circuit as in FIG. 2, would also be possible in the sense that a resistance is interconnected between the base of each transistor Q₁, Q₂ and the assigned capacitor C₃₁, C₃₂ and that under certain circumstances the emitter resistors R₁₁, R₁₂ are omitted. In this latter case the transistors Q₁, Q₂ are easily saturated, and it will then be necessary to dimension the frequency of the triggering pulse train or the circuit such, that the transistors Q₁, Q₂ will remain conductive until even the larger of the capacitors C₃, C₄ is fully charged, whereby the differential amplifier then responds essentially via the bridge arms to the variable duration of the currents.

In the embodiments shown by way of example, the bridge arms always have two capacitors, but that is not absolutely necessary, and it is also possible always to replace one by resistors. 

We claim:
 1. A capacitive measuring circuit, comprising:first and second bridge arms each including at least one capacitor; a differential amplifier having an input connected to each of said bridge arms and an output fed back to at least one of said first or second bridge arms; a common oscillator being connected to said first and second bridge arms via one switching element each, each said switching element having two terminals, one terminal being connected to said oscillator and the other one to said respective bridge arm, said oscillator having an output for providing a rectangular pulse train voltage to said switching elements; a number of potential sources; a first passive circuit element for connecting one of said potential sources to a respective one of said first and second bridge arms, said potential source being connected to the other terminal of said switching element connected to said bridge arm; a second passive circuit element for connecting a second potential source to the other of said two bridge arms, said second potential source being connected to the other terminal of said switching element connected to said other of said two bridge arms; the rectangular pulse train applied to said switching elements connected to said bridge arms oscillates between a potential higher and a potential lower than said potentials applied to said capacitors of said bridge arms from said potential sources via said passive circuit elements shifted by the amount of the passing through voltage of said switching elements to allow said capacitors to be charged and discharged alternately whereby the duration of the pulses and intervals of the pulse train allows that the charging current of the capacitors drops practically to zero; a feedback circuit including only second passive circuit elements only connected with at least one capacitor of one of said two bridge arms via one of said first and second passive elements and with the output of said differential amplifier.
 2. Measuring circuit as claimed in claim 1 wherein said switching elements are diodes and said pulse train output of said oscillator fluctuates between fixed values which are respectively above and below the potential, shifted by the forward voltage of said diodes, at the connecting points of the diodes with said first and second bridge arms.
 3. Measuring circuit as in claim 1 or 2 wherein each of said first and second bridge arms includes two capacitors and further comprising amplifiers having outputs connected with the respective inputs of said differential amplifier.
 4. Measuring circuit as in claim 3 further comprising a fixed potential source and additional diodes connecting said fixed potential source to the connecting points of said first and second bridge arms.
 5. Measuring circuit as in claim 4 wherein said amplifiers each include a transistor having an emitter and a base electrode connected to the respective connecting point of said first and second bridge arm and further comprising a resistor interconnecting a respective emitter of said transistors to said fixed potential source.
 6. Measuring circuit as claimed in claim 1 further comprising an inverter for inverting the output of said differential amplifier; one of said first and second bridge arms being connected to said output of said differential amplifier via said feedback circuit including one of said first and second passive elements respectively and the other one of said bridge arms being connected to said inverter via the other one of said first and second passive elements respectively.
 7. Measuring circuit as claimed in claim 1, said differential amplifier being provided with a second output delivering output signals inverse to that one of said first output, one of said first and second bridge arms being connected to said first output of said differential amplifier via said feedback circuit including one of said first and second passive element respectively and the other one of said bridge arms being connected to said inverting output of said differential amplifier via the other one of said first and second passive element respectively. 